As mentioned previously, a good portion of recent studies of NGC 2903 are focused on the nuclear region, due to its hotspot nucleus. Though NGC 2903 is hardly the only such galaxy to have nuclear hotspots. In 1973, J. L. Sersic published "A List of Galaxies with Peculiar Nuclei" (Sersic, 1973) after studying the Hubble Plate Collection at Hale Observatories (Sersic, 1973). This lists about 31 galaxies with hotspot nuclei.
The earliest paper I could find that focused on this area was written by Oka, et al. in 1974. They performed many spectrographic and photometric observations of the nuclear region in order to determine the properties of the hot-spots. They found that the spectra were consistent with the HII regions (emission nebulae/star forming regions) in the spiral arms. (Oka, Wakamatsu, Sakka, Nishida, & Jugaku, 1974) They also found evidence for both early and late type stars within the hotspots, and concluded that the hot-spots were young giant clusters made of both early and late type stars in ionized gas, ie, giant HII regions.(Oka, Wakamatsu, Sakka, Nishida, & Jugaku, 1974) The following sections summarize some more recent research papers on the object.______________________________________________
In a study performed by Simons et al (1988) a CCD was used to capture the images, as opposed to the photographic plates of the earlier studies, and had somewhat better resolution, about 1"(D.A, D.L, E.E, R.W, K.-W, & D.N.B, 1988). As in the previous studies, they performed several photometric and spectrographic measurements. They also did something called colour mapping. Essentially, they took a K-band (IR) image and divided it by a V-band (green) image. Bright areas on the resulting image were redder and the dark areas were more blue(D.A, D.L, E.E, R.W, K.-W, & D.N.B, 1988). They also summarized previous results from other papers, stating that the hot spot area can be described best as an "energetic star-formation region" (D.A, D.L, E.E, R.W, K.-W, & D.N.B, 1988). From their own results they determined that extinction was the main cause in colour difference (D.A, D.L, E.E, R.W, K.-W, & D.N.B, 1988), and posited that the hot spots were actually holes in the distribution of dust across a large star-formation area in the nucleus (D.A, D.L, E.E, R.W, K.-W, & D.N.B, 1988).
Infrared array imaging and spectrophotometry of the nuclear region of the 'hot-spot' galaxy NGC 2903
D. A. Simons, D. L. Depoy, E. E. Becklin, R. W. Capps, K.-W. Hodapp, and D. N. B.Hall
Abstract
Previous studies of the 'hot-spot' galaxy NGC 2903 at a variety of wavelengths have shown that considerable star formation is occurring within the complex hot-spot structures in this galaxy's nuclear region. A K-band (2.2 microns) image of the galaxy's nuclear region has been obtained with an infrared CCD and compared to V-band (0.55 micron) and 1-micron images of the same region. The data show that changes in hot spot morphology in the V band are due primarily to variations in extinction, though variations in stellar populations and densities are also probably important. The K-band/V-band image ratio has been used to map the variation of color across the inner about 20 arcsec of the galaxy. The structures revealed in this image ratio suggest that a dust lane extends through the nucleus. Spectrophotometry of the Br-alpha hydrogen recombination line emission from two positions within the imaged region has also been acquired. The measured line strengths show that ionized gas and active star formation exist well outside of the hot spots.
From NASA Astrophysics Abstract Service -- 1988ApJ...335..126S
This is a SDSS g filter image of the nucleus of NGC 2903 blurred to about the amount of detail they used in the study:
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~ 18"x12" |
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In 2001, a study of the central region using the Hubble Space Telescope/NIMCOS and ground-based spectroscopy resolved the IR hot spots into individual star clusters for the first time (Anonymous 2003). Until then, previous studies in the near infrared didn't have a high enough resolution to determine whether the star formation was confined to the hot spots or not.(A, S.D, & J.H, 2001) When they examined their data, they discovered that the HII regions did not coincide with the stellar clusters (A, S.D, & J.H, 2001) like previous studies had assumed for lack of better resolution. Instead, the HII regions have a ring-like structure with a diameter of about 625 pc.(A, S.D, & J.H, 2001) (a more detailed relation to my research question is discussed in the discussion section)
Nuclear star formation in the hot-spot galaxy NGC 2903
A. Alonso-Herrero, S. D. Ryder and J. H Knapen
Abstract
We present high-resolution near-infrared imaging obtained using adaptive optics and HST/NICMOS, and ground-based spectroscopy of the hotspot galaxy NGC 2903. Our near-infrared resolution imaging enables us to resolve the infrared hotspots into individual young stellar clusters or groups of these. The spatial distribution of the stellar clusters is not coincident with that of the bright Hii regions, as revealed by the HST/NICMOS Paα image. Overall, the circumnuclear star formation in NGC 2903 shows a ring-like morphology with an approximate diameter of 625pc. The star formation properties of the stellar clusters and Hii regions have been studied using the photometric and spectroscopic information in conjunction with evolutionary synthesis models. The population of bright stellar clusters shows a very narrow range of ages, 4-7×106yr after the peak of star formation, or absolute ages 6.5-9.5×106yr (for the assumed short-duration Gaussian bursts), and luminosities similar to the clusters found in the Antennae interacting galaxy. This population of young stellar clusters accounts for some 7-12per cent of the total stellar mass in the central 625pc of NGC 2903. The Hii regions in the ring of star formation have luminosities close to that of the supergiant Hii region 30 Doradus, they are younger than the stellar clusters, and they will probably evolve into bright infrared stellar clusters similar to those observed today. We find that the star formation efficiency in the central regions of NGC 2903 is higher than in normal galaxies, approaching the lower end of infrared luminous galaxies.
From NASA Astrophysics Abstract Service -- 2001MNRAS.322..757A
This is one of the images used in the study. It is a MAST HST image (imported via Aladin) through the F160W filter of the NIC2 (this filter corresponds to the H-band of infrared) (A, S.D, & J.H, 2001) the field of view is ~19x19 arcminutes
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The most recent paper on NGC 2903 that I could find was based on X-ray observations of the halo region, attempting to ascertain whether there were starbursts in the nucleus. A starburst is a region where a large amount of star formation is taking place(Wikipedia, ) and usually last about a few 10^7 years.(D, G, & N, 2003) Massive OB type stars (hottest types) end in type II supernovae. If there are a lot of them forming in one area at the same time, then they will all go nova at around the same time. The Supernovae build up a lot of hot plasma and can expand off the disk into the halo region if they have enough kinetic energy.(D, G, & N, 2003) This shows up in X-rays and are referred to as "superwinds"(D, G, & N, 2003) Studying their results, they found that there is a definite extended X-ray gas component that extends about 5 kpc above the disk (D, G, & N, 2003). Since NGC 2903 is isolated, (no companion) they determined that the filament is not due to an external element.(D, G, & N, 2003)
An X-ray halo in the "hot-spot" galaxy NGC 2903
D. Tschoke, G. Hensler, N. Junkes
Abstract
In this paper we present ROSAT PSPC and HRI observations of the ``hot-spot'' galaxy NGC 2903. This isolated system strikingly reveals a soft extended X-ray feature reaching in north-west direction up to a projected distance of 5.2 kpc from the center into the halo. The residual X-ray emission in the disk reveals the same extension as the Hα disk. No eastern counterpart of the western X-ray halo emission has been detected. The luminosity of the extraplanar X-ray gas is several 1038 erg s-1, comparable to X-ray halos in other starburst galaxies. It has a plasma temperature of about 0.2 keV. The estimated star formation rate derived from X-rays and Hα results in 1-2 M_sun yr-1. Since galactic superwinds, giant kpc-scale galactic outflows, seem to be a common phenomenon observed in a number of edge-on galaxies, especially in the X-ray regime, and are produced by excess star-formation activity, the existence of hot halo gas as found in NGC 2903 can be attributed to events such as central starbursts. That such a starburst has taken place in NGC 2903 must be proven. The detection of hot gas above galaxy disks also with intermediate inclination, however, encounters the difficulty of discriminating between that contribution from disk and active nuclear region.
From NASA Astrophysics Abstract Service -- 2003A&A...411...41T
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Banner image is a slice of HST image by NASA/ESA. Article abstracts taken from NASA Astrophysics Abstract Service